Alkalinity determination by potentiometry: intercalibration using three different methods

Abstract

Recibidü en marzo de 1999. aceptado en ~e~i r e m b r e de i999 ABSTRACTSeawater was sarnpled from different depths in the North Atlantic Ocean (Canary Islands region) and distributed among three different labs for the determination of titration alkalinity. Analysis was performed by potentiornetric rnethods. involving titration in a closed cell, titration in an open cell and a two end-point acid addition method. The precisíon, which is the sarnple reproducibility taken from the mean sranaarci aeviarion Íor repiicate measurements, was ~erween 0.45 anci 0.90 prnoi icg-' for ine individual labs. Accuracy, here taken as the deviation for the values of a lab from the rnean of al1 three, was mostly below 1 pmol -kg-' and never exceeded O. 1% of the sarnple value. Mean standard deviation for al1 labs and al1 sarnples was 0.87 prnol . kg-', once the individual methods were calibrated using Ciencias Marinas, Vol. 26, No. 1,2000 certified reference material (CRM). Without CRM calibration, the mean standard deviation would increase to 2.8 p n o l . kg-'. The conclusion is that current high precision rnethods for alkalinity measurements calibrated with CRMs are able to reach similar accuracy as the measurement of total dissolved inorganic carbon by coulornetry and therefore allow for the precise determination of the oceanic carbon dioxide system by using the two measured parameters.Key words: alkalinity, potentiometry, seawater, certified reference material RESUMEN Se determinó la alcalinidad de muestras de agua de mar recogidas a diferentes profundidades en el Océano Atlántico Norte y distribuidas entre tres laboratorios, utilizando tres variantes potenciométricas: valoración en cubeta cerrada, valoración en cubeta abierta y un método de adición a doble punto final. La precisión, la reproducibilidad de la muestra obtenida a partir de la desviación estándar media para medidas replicadas, fue entre 0.45 y 0.90 p o l . kg-' para cada laboratorio. La exactitud, tomada como la desviación de los valores de un laboratorio respecto de la media de los tres, fue mayoritariamente menor que 1 pmol . kg-' y nunca excedió 0.1% del valor de la muestra. La desviación media estándar para todos los laboratorios y todas las muestras fue de 0.87 pmol . kg-', una vez que los métodos individuales se calibraron usando material certificado de referencia (CRM). Sin la calibración con CRM, la desviación estándar media se incrementaria a 2.8 pmol . kg-'. Se concluye que la precisión real de los métodos para la medida de la alcalinidad calibrada con CRM es capaz de alcanzar exactitudes similares a las medidas de carbono inorgánico total por coulometría y por lo tanto permite la determinación precisa del sistema de carbono oceánico usando la medida de las dos variables.Palabras clave: alcalinidad, potenciometría, agua de mar, material certificado de referencia.

Inorganic carbon and pH levels in the Rockall Trough 1991–2010

“…(A T ) 2009 and 2010 C T and A T were sampled and analysed as per Dickson et al (2007). C T was analysed by coulometric titration and A T by potentiometric titration at NUI, Galway, both on a VINDTA-3C (Versatile INstrument for the Determination of Total inorganic carbon and titration Alkalinity), manufactured by Marianda (Mintrop et al, 2000). The system was calibrated by running duplicate Certified Reference Materials with every batch of samples (CRMs provided by A.…”

Marine ecosystem community carbon and nutrient uptake stoichiometry under varying ocean acidification during the PeECE III experiment

“…Following the pCO 2 measurements, using the same sampling methodology, samples for total alkalinity (A T ) and total dissolved inorganic carbon (C T ) were drawn into 500 ml borate bottles and immediately poisoned with HgCl 2 . A T was measured using Gran potentiometric titration (Gran, 1952) on a VINDTA system (Mintrop et al, 2000) with a precision of ≤4 µmol kg −1 . C T was determined using coulometric titration (Johnson et al, 1987) with a precision of ≤2 µmol kg −1 .…”

Trends in anthropogenic CO2 in water masses of the Subtropical North Atlantic Ocean

“…The pH was measured spectrophotometrically (Clayton and Byrne, 1993) and A T was determined potentiometrically by titration at endpoint detection (Mintrop et al, 2000). A T gaps were filled as reported above.…”